A photon detector detects photons, such as an x-ray beam, that are transmitted from a source to a detector. While this invention is primarily explained using computed tomography (CT) detectors as an example, the invention is also directed to other detectors for medical imaging devices using photon detection, such as other medical imaging devices, for example positron emission tomography device, single-photon emission computed tomography, security scanning devices, optical imaging devices, such as digital cameras; radiographic imaging devices and astronomical devices.
In object scanning apparatuses, such as medical and security imaging apparatuses an object to be scanned, e.g. a patient or luggage, is positioned in an examination region between the photon source and the detector. In other photon detection apparatuses, such as cameras or astronomical apparatuses, a (mostly) unobstructed photon beam may be detected. A detector usually comprises detector pixels and a very precise characterization of each individual pixel is required to evaluate the scanning data and generate high quality images. A response function of each detector pixel is determined to characterize the behavior of the detector pixel by describing the probability distribution of photon pulse heights detected by the pixel. For photon counting detectors, the response function describes the probability distribution of pulse heights for each energy of incoming photons detected by the pixel. The response function is determined during a calibration procedure. Radiation spectra employed in this calibration procedure are preferably monochromatic spectra, since then the response function may be measured directly using a threshold scan, i.e., a number of acquisitions where the energy threshold values of the detectors are varied from acquisition to acquisition. However, monochromatic sources (such as synchrotron radiation or gamma-sources) are not suitable for a calibration of imaging apparatuses with photon counting detectors. Therefore, one has to realize the calibration using the photon source in place. In order to shape a continuous photon (e.g. x-ray) spectrum from the source it has to be filtered with absorber materials with high atomic numbers having absorption edges (K-edges) at relevant energies. For example, a data acquisition with a nearly monochromatic spectrum can be approximated by performing two measurements with k-edge filters having slightly different K-edge energies and calculating the differences of both individual measurements.
For optimal calibration specific materials, such as rare earth metals or elements with high atomic numbers, have to be used, for example the K-edge energies of rare earth metals span the important energy range from 39 to 63 keV. A drawback is that it is very difficult to produce homogeneous samples of these metals or other elements with higher atomic numbers. Since a high image quality can only be achieved if the calibration steps lead to very precise results, small inhomogeneties of the mass thicknesses of K-edge filters, either due to thickness variations, holes or inclusions, will lead to differences in the calibration data of neighboring pixels. These differences may lead to image artifacts.
It would therefore be beneficial to overcome the above stated disadvantageous dependence on high homogeneity of K-edge filters for photon counting detector calibration.
US20110012014 relates to detecting radiation that traverses a material having a known spectral characteristic with a radiation sensitive detectr pixel that outputs a signal indicative of the detected radiation and then determining a mapping between the output signal and the spectral characteristic. It does not provide any way of dealing with the above stated disadvantageous dependence on high homogeneity of K-edge filters for photon counting detector calibration
Robert E. Alvarez (MEDICAL PHYSICS, AIP, MELVILLE, N.Y., US, vol. 38, no. 5, pages 2324-2334, published Apr. 25, 2011) disclose a CT calibration phantom with different dimensions configured so that the gantry can make measurements of different projections of the phantom.